In order to make aircraft passengers comfortable, and in order to transport them between an airport terminal building and an aircraft in such a way that they are protected from the weather and from other environmental influences, passenger boarding bridges are used which are telescopically extensible and the height of which is adjustable. For instance, an apron drive bridge in present day use includes a plurality of adjustable modules, including: a rotunda, a telescopic tunnel, a bubble section, an aircraft-engaging cabin, and elevating lift columns with wheel carriage. Typically, one lift column is mounted adjacent to each lateral surface of the telescopic tunnel. Other common types of passenger boarding bridges include radial drive bridges and over-the-wing (OTW) bridges. These types of passenger boarding bridges are adjustable, for instance to compensate for different sized aircraft and to compensate for imprecise parking of aircraft at an airport terminal.
The lift columns are for adjusting the height of the passenger boarding bridge, so as to position the cabin at a proper height above the ground for engaging a doorway of an aircraft. After the cabin is aligned with the doorway, the lift columns are used to support the passenger boarding bridge in such a way that an approximately level walking surface is maintained between the doorway of the aircraft and the cabin of the passenger boarding bridge. In particular, the aircraft rises and lowers on its undercarriage as it is first unloaded of passengers, baggage and cargo, and then reloaded and fueled for the next flight. For this reason, passenger boarding bridges typically are equipped with autolevel mechanisms for sensing vertical movement of the aircraft and for automatically adjusting the height of the cabin. Accordingly, the lift columns are activated from time to time while the aircraft is being loaded and unloaded, in order to compensate for the vertical movement of the aircraft.
Typically, each lift column is provided with a separate mechanism for varying a length thereof. The mechanism optionally is electrohydraulic in nature, as where a motor drives a pump to supply fluid for extending and retracting a hydraulic cylinder, or is electromechanical in nature, as where a motor drives an electromechanical screw. In either case, the motor is responsive to a control signal for raising and lowering the outboard end of the passenger boarding bridge. For instance, in the case of an electromechanical screw a first control signal operates the motor in one direction and causes the mechanism to elevate the outboard end of the passenger boarding bridge, and a second control signal reverses the motor and causes the mechanism to lower the outboard end of the passenger boarding bridge.
In the case of electromechanical screw mechanisms, the motors must turn the screws of both lift columns at the same rate (to within a threshold limit) in order to successfully raise and lower the passenger boarding bridge. If the screws are turned at different rates, then a rack fault condition occurs in which the passenger boarding bridge twists between the two lift columns. The rack fault condition not only prevents further adjustment of the passenger boarding bridge, but may also cause damage thereto.
In the past, limit switches have been disposed one each on the lift columns of passenger boarding bridges. The limit switches are mechanical sensors, which are activated when one of the lift columns is moving slower relative to the other lift column. When activated, the limit switches provide a control signal for disabling further vertical adjustment of the passenger boarding bridge and for displaying an error message that is indicative of a rack fault condition having occurred. It is a disadvantage that the limit switches merely detect the rack fault condition after it has occurred. In particular, no further vertical movement of the passenger boarding bridge is possible until the rack fault condition is corrected, thereby causing a delay in disembarking the passengers that are aboard the aircraft. If the delay cannot be overcome then the next flight will not leave on time, which inconveniences the passengers and is costly for the airline.